Many types of ground drilling systems are available for drilling holes for particular purposes and in specific ground conditions. One range of downhole drill systems utilise a fluid under pressure to assist in advancing the drill. The fluid may act to either drive a drilling tool coupled to an associated drill string, or to flush drill cuttings from a hole being drilled, or both. The fluid can be a gas such as air or nitrogen, a liquid/slurry such as water or drilling mud, or a combination of a gas and liquid.
For oil and gas exploration and production it is common to use downhole motors which are driven by high specific gravity fluid such as drilling mud to provide rotation to an attached roller bit. The mud can also act to clear cuttings from the hole and provide downhole pressure control. Additionally the volumetric flow rate of mud through a mud motor may be sufficient to kill a well if required. However there is a limitation in terms of drilling in hard materials particularly with directional (i.e. non-vertical holes). This arises due to the inability to apply sufficient downhole pull-down or weight on bit (“WOB”) to fracture rock and progress the drilling at an economical rate.
The limitation of penetration in hard materials can overcome by the use of a down the hole (DTH) hammer. DTH hammers are driven by a fluid. While air is a common driving fluid it does not enable control of downhole and ground pressure. Also it is often not possible to provide the air with the required pressure and volume to provide sufficient pressure differential with reference to the prevailing down hole environment to effectively drive the hammer.
Instead of air, water and additives such as drilling mud can used to drive the hammer. This enables higher drilling pressures to be provided to combat high ground pressures. However due to its inherent nature drilling mud rapidly wears the internal surfaces of the hammer leading to the need for frequent replacement. This involves the very time consuming process of tripping the drill string. Also conventional hammer drills do not enable a sufficient volumetric flow rate to kill a well (i.e. flood the well quickly to control or stop the flow of gas and other dangerous well conditions) in the event of a dangerous over pressure condition.